This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 101 57 849.0, filed on Nov. 24, 2001, the entire disclosure of which is incorporated herein by reference.
The invention relates to an arrangement for reducing the aerodynamic noise generated by a leading edge slat of a main wing of a commercial passenger transport aircraft.
Modern commercial passenger transport aircraft are equipped with high-lift auxiliary devices that are typically deployed during take-off and/or landing phases of a flight in order to increase the lift generated during relatively low flight speeds. These high-lift auxiliary devices include leading edge slats and landing flaps, which are respectively movably connected to the leading edge area and the trailing edge area of a main wing, so as to be selectively extendible from or retractable to the main wing. In the extended or deployed positions, these high-lift auxiliary devices, in addition to the extended landing gear, represent the major generators of aerodynamic flow noise of commercial transport aircraft. For example, typical leading edge slats used on modern commercial transport aircraft are of the Handley Page slat type, which forms an air gap or slot between the slat and the forward nose of the main wing. The air flowing through this gap or slot achieves the desired increase of the generated lift, but simultaneously also leads to an increased noise generation. This aerodynamically generated slat gap noise can actually reach or exceed the noise level magnitude of the jet engines, when the engines are sharply throttled back during a landing approach.
In view of the above, it has long been a serious problem and the subject of substantial research in the field of aircraft design, to reduce the aerodynamically generated noise of the air flowing over various aircraft structures, and especially the extended high-lift auxiliary devices, and particularly the extended leading edge slats. For example, results of a flight research program of the Federal Republic of Germany have shown that the leading edge slat contributes a higher proportion of the total noise, in comparison to the noise generated by the landing flap. Detailed studies have identified a well-developed entrapped eddy vortex in the airflow on the concavely curved rear surface or inner surface of the leading edge slat facing the forward nose surface of the main wing. This entrapped eddy vortex is a significant potential noise source.
The noise generation of this entrapped eddy vortex is understood as follows. A flow separation of the gap airflow constantly occurs between the slat and the main wing along the above mentioned concavely curved inner surface of the extended slat, and thus generates the entrapped eddy vortex. This vortex is continuously supplied with energy by the accelerated gap airflow bordering along the slat. Also, small turbulence cells are continuously formed along the boundary or flow separation line between the vortex flow area and the continuous gap airflow flowing through the gap. These turbulence cells continuously become entrained in the accelerated gap flow, whereby the major noise is generated, especially due to the further flow of these turbulence cells past the upper rear or trailing edge of the slat and then over the upper surface of the main wing.
A study by Dr. Werner Dobrzynski, Mr. Burkhard Gehlhar, and others, entitled xe2x80x9cAirframe Noise Studies on Wings with Deployed High-Lift Devicesxe2x80x9d, Deutsches Zentrum fuer Lift- und Raumfahrt e.V. (DLR), Institut fuer Entwurfsaerodynamik, Abteilung Technische Akustik, Forschungszentrum Braunschweig, Germany, published in the American Institute of Aeronautics and Astronautics, 4th AIAA/CEAS Aeroacoustics Conference, Jun. 2-4, 1998, Toulouse France, is also directed to the reduction of aerodynamic noise on an extended leading edge slat of an aircraft. Among other things, this study investigates a possible solution to the noise problem, which involves an airflow guide plate that is hingedly secured to the leading edge slat in the area of the inner or rearward profile area thereof and extends in a direction toward the main wing in the airflow direction. This airflow guide plate is hinged and can thus be pivoted inwardly relative to the leading edge slat. This solution aims to reduce the noise level during take-off and landing of an aircraft with extended slats. When the slat is retracted for cruise flight, the guide plate is then pivoted inwardly against the slats.
Although the above described arrangement of a hinged airflow guide plate may have achieved noise reductions in wind tunnel tests, this solution is not expected to find substantial use in real world applications, in view of practical considerations and difficulties in the actual practice thereof. For example, in the retracted condition of the leading edge slat, e.g. the cruise configuration, the guide plate must be pivoted or tilted against the rearward profile surface of the leading edge slat, and must then have a contour or configuration that is sufficiently matched to the rear curvature of the slat. However, that is not the proper curvature contour of the guide plate for its operation. Furthermore, the retracted position of the slat does not provide sufficient space to allow such a rigidly configured guide plate to be stored between the retracted slat and the nose area of the main wing. On the other hand, if the guide plate is to be flexible, to adapt itself to the curvature of the available space in the retracted and stowed condition of the slat, then such a flexible guide plate would not have sufficient strength and stiffness to durably withstand the significant aerodynamic forces that arise from the airflow through the slat gap in the extended condition of the slat. As a result, the guide plate will tend to flutter, with the end result of radiating noise, which is directly contrary to the intended noise reduction effect.
Furthermore, a pivotally connected or hinged guide plate requires additional mechanically movable parts, which disadvantageously lead to an increase of the manufacturing, installation, maintenance and repair costs, as well as an increase of the total installed weight in the aircraft. Another problem is that the transition from the lower surface of the slat to the hinge of the guide plate or separation surface must be free from contour discontinuities or jumps as well as open slots, which therefore requires very high fabrication and installation accuracy with low tolerances.
Another problem is that the metal guide plate or separation surface is subjected to considerable alternating forces that are initiated by the airflow. Since this guide plate or separation surface is connected only to the bottom edge of the slat via the hinge joint, and no further supports or stiffening arrangements are provided, there is a significant danger that the guide plate or separation surface will be stimulated to oscillate or vibrate back and forth. That would cause significant airflow disruption, drag, and additional noise. Furthermore, since the contour of the rear surface of the slat, as well as the geometry of the air gap, varies over the span of the wing, the various elements of this guide plate or airflow separation surface must be formed with a taper or angled inclination over the span, which leads to additional complication of the retraction mechanism. The situation of any fault or failure becomes especially critical, for example if the mechanism becomes blocked, because then the slat can no longer be retracted.
The above cited publication gives no suggestions or motivations toward overcoming or avoiding these disadvantages, or toward any other device or arrangement that might achieve a better overall result without suffering such disadvantages.
The German Patent Publication DE 199 25 560 A1 aims to reduce the above discussed aerodynamic noise by installing a massive separating member forming a separation surface that is movable relative to the slat, for example in the manner of a formed metal plate along the airflow boundary or separating line between the above mentioned entrapped eddy vortex and the slat gap airflow. Nonetheless, the above discussed disadvantages would also apply to such an arrangement. Additionally, there is a danger that such a covering of the concave curved inner surface or rear surface of the slat will form a resonance volume, which will actually lead to an increased noise radiation. Insofar as this formed metal plate, which is movably connected to the inner bottom edge of the extended slat, does not completely cover, enclose and separate the rear inner surface of the slat and therewith the entrapped eddy vortex, it must be assumed that the gap flow that flows between this metal plate and the nose curvature of the main wing will separate and become turbulent along the rear free edge of the metal plate, which will energize and excite the separated air forming the entrapped eddy vortex to undergo resonance oscillations, which in turn will generate a low frequency noise.
Furthermore, the German Patent Publication DE 100 19 185 A1 and corresponding U.S. Pat. No. 6,394,396 (Gleine et al.) disclose an arrangement for reducing the aerodynamic noise of a leading edge slat of the main wing of a commercial passenger transport aircraft, including a hollow expandable and contractible displacement element secured onto the concave rear surface of a slat facing the leading edge of the aircraft wing. A bleed air line supplies engine bleed air through a suitable control arrangement into the hollow displacement element to selectively expand or contract the displacement element. When the slat is extended, the displacement element is expanded to fill-out the concave cavity on the rear surface of the slat so as to prevent the formation of an entrapped eddy vortex in the slat air gap, and thereby reduce the generation of aero-acoustic noise. When the slat is retracted, the displacement element is contracted to be conformingly accommodated in the sickle-shaped space between the slat and the leading edge of the wing. While this arrangement achieves a significant reduction of the aerodynamically generated noise, the system requires an active control arrangement, and the expandable displacement element is subject to aging degradation as well as wear and the like.
It would thus be desirable to develop a system that has lower inspection and maintenance requirements, a higher durability and reliability, and that does not require an active control arrangement.
In view of the above, it is an object of the invention to provide an arrangement for reducing aerodynamically generated noise on a leading edge slat on a main wing of a commercial transport aircraft, with a simple, lightweight structure and arrangement, and a simple installation and retrofitting capability, and without negatively influencing the aerodynamic characteristics such as lift and drag of the overall wing structure. Moreover, in the event of a failure of any component of the arrangement, there must be no dangerous effects on the further proper and safe operation of the slat and the aircraft overall. Thus, the invention further aims to avoid or minimize the use of additional movable mechanical components and actively actuated components. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification. The attainment of these objects is, however, not a required limitation of the claimed invention.
The above objects have been achieved according to the invention in a wing arrangement for an aircraft, including a wing body having a forward nose, and a slat arranged with its concave rear surface facing toward the forward nose of the wing body, whereby the slat is movably connected to the wing body to be movable between a retracted slat position with the concave rear surface of the slat facing and adjacent to the forward nose of the wing body, and an extended slat position in which the slat is spaced away from the forward nose with a slat air gap bounded between the slat and the forward nose. The inventive arrangement further particularly comprises plural bristles, fibers, hairs, threads, or the like, generally called xe2x80x9cbristlesxe2x80x9d herein, which are distributed to form at least one row along the lower rearward edge and/or the upper rearward edge of the slat in the span direction.
The bristles are preferably flexible so that they are self-adjusting or self-contouring by the aerodynamic forces exerted on them by the respective prevailing airflow conditions, so that the row of bristles forms a smoothly contoured separation between the entrapped eddy vortex on the rear concave side of the slat, and the smooth gap airflow through the slat gap. Due to their flexibility, when the slat is retracted, the flexible bristles will also easily deform to be received and stowed in the sickle-shaped space remaining between the concave rear surface of the slat and the convex forward surface of the leading edge nose of the wing body.
Additionally, to facilitate the self-orienting movement of the flexible bristles in the extended condition of the slat, and the self-stowing movement of the bristles in the retracted condition of the slat, the base ends of the bristles may be hingedly connected to the lower rear edge or the upper rear edge of the slat, for example by means of a carrier element extending longitudinally in the span direction along the slat. This enables a pivoting or hinging movement of the bristle arrangement overall, in addition to the flexible self-contouring of each individual bristle.
The bristles are preferably provided and arranged with a sufficient number and density of bristles to form an aerodynamically effective separation surface between the entrapped eddy vortex and the smooth or laminar gap airflow as mentioned above. However, the density of the bristles is limited so that the bristles preferably do not form a closed or airtight separation surface, but instead allow a limited air permeability through the airflow separation surface, to provide a controlled pressure compensation between the entrapped eddy vortex and the smooth gap airflow on opposite sides of the separation surface. The limited air permeability through the separation surface formed by the bristles achieves a gradual or smooth compensation of the turbulent alternating pressure conditions that exist between the entrapped eddy vortex and the gap airflow in the flow direction thereof. The overall result is a substantial reduction of the aerodynamically generated noise in the slat gap area.